Hunt for Higgs boson: Mass of top quark narrows search

Dec 07, 2009

(PhysOrg.com) -- New high-energy particle research by a team working with data from Fermi National Accelerator Laboratory further heightens the uncertainty about the exact nature of a key theoretical component of modern physics -- the massive fundamental particle called the Higgs boson.

Analysis of data from particle collisions resulting in two leptons helps improve measurements of the mass of another heavy subatomic particle called the top quark, says physicist Robert Kehoe at Southern Methodist University, who led the team that calculated the measurement. Improving the measurement of the mass of the top quark bears on the nature of the Higgs, says Kehoe, an assistant professor in SMU's Department of Physics.

The Higgs was postulated in the 1960s to help explain how basic elements of the universe fit together and interact. It is responsible for a phenomenon called the Higgs mechanism, which gives mass to the fundamental particles of nature. Physicists have searched for more than four decades to observe the never-before-seen Higgs. Now they hope it will be observed in the next few years since data started flowing recently from the world's newest and largest high-energy particle accelerator, the CERN Large Hadron Collider near Geneva, Switzerland.

Physicists theorize that the top quark — because of its sizable mass — is sensitive to the Higgs and therefore may point to it. They theorize that knowing the mass of the top quark narrows the range of where the Higgs will be detected in CERN's LHC collisions. The top quark is one of 16 species of subatomic particles that physicists have observed. It was predicted in the 1970s and observed in 1995. Increasingly precise measurements of its mass have been achieved almost every year since, and physicists closely watch the incremental measurements of the top quark.

The two-lepton analysis by Kehoe and SMU post-doctoral researcher Peter Renkel looked at data taken over four years during high-energy collisions at Fermilab, a Department of Energy proton-antiproton collider in Batavia, Ill. The two-lepton analysis is one of almost a dozen analyses of the mass of the top quark at a Fermilab experiment called DZero." The DZero experiment involves 500 physicists and is one of Fermilab's two large experimental collaborations of scientists. The top quark mass was first observed simultaneously by these two experiments. Several measurements of the top quark's mass from these two experiments are combined to a "world average" value.

The two-lepton analysis contributed to the latest world average measurement. The analysis looked at particles resulting from smashing protons that break apart and disintegrate. The events are very rare, and the detector can't see two of the important "ghost" particles — neutrinos — produced by the collision. However, the two leptons are well-measured events and are not seen in other "background" collisions where top quarks are not produced. This allows a rapidly improving precision to be achieved.

The two-lepton research was published in November in the article "Measurement of the top quark mass in final states with two leptons" in Physical Review D, the American Physical Society's journal of particles, fields, gravitation and cosmology. SMU physicists collaborated on the research with scientists at Boston University. The SMU portion of the work was funded by the Department of Energy.

Standard Model fundamental particles. Source: Fermilab

The new world average is so precise that it constrains more tightly than ever the range of possible measurements for the mass of the Higgs, Kehoe says. If the Higgs does prove different than currently expected, physicists may have to rework their long-standing theoretical framework, known as the Standard Model. Scientists worldwide are hoping to validate the Standard Model — which has worked well for more than 30 years to explain everything from radioactivity to computer chips — by actually observing the Higgs.

"The new results may be an indication that the Higgs boson has different properties than the Standard Model indicates," Kehoe says. "It's very difficult to devise a theory without some mechanism that mimics fairly well the Higgs mechanism. But if the underlying cause of this mechanism is significantly different, that will have a major impact on the fundamentals of the Standard Model. It could point to something deeper than the standard Higgs boson at work, and that is very interesting."

The measured value of the top quark mass may even go beyond constraining the standard Higgs. It may suggest that our current understanding of the Higgs is not correct, he says.

If the Higgs does not show up where the constraints indicate, the top measurement may force consideration of new theoretical possibilities that lie outside the existing Standard Model, Kehoe says.

Previous measurements have put the top quark at almost the mass of a gold atom. The new world average measurement puts the top quark at about 186 times the mass of the proton. While the value has changed only a small amount from previous measurements, the percentage of error on the measurement is progressively smaller, in this case less than 1 percent.

"If we make a precise prediction of where the Higgs is and it's not there, then something is wrong. We've just found a major flaw in the model," says Kehoe, whose work has focused for 16 years on the top quark, including as a graduate student on DZero working directly on the discovery analysis. "It would tell us that the model is oversimplified and that reality is much more complicated."

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The top quark is not a constituent of the Hydrogen atom (*). The protons and neutrons that make up the nuclei of atoms are composed primarily of up and down quarks, plus the massless gluons that bind them together. Top quarks are only produced in very high energy interactions.

(* well, someone may mention loops and such. The valence model of protons and neutrons is what I am referring to.)

"Previous measurements have put the top quark at almost the mass of a gold atom. The new world average measurement puts the top quark at about 186 times the mass of the proton. "

Now I am EXTREMELY confused...How can a top quark, one of the constituents of a hydrogen atom, have about 186 times the mass of a hydrogen atom? Anyone? Anyone?

Theoretical Physics is the essence of recycling. If what you have is not working for you anymore, change it into something new and ask the government to fund you through the transition ...

Now that gold has a mass hundreds of times its weight we have discovered antigravity and eliminated the need for Dark Matter, and also recovered the entire National Treasury in one 'sweeet' move. More money, please.

Jokes aside, The (new) Emporer's New Clothes are wearing exceptionally thin with this one. Is it just a Red Herring for the LHC to waste their time and money on?

@gunslingerNo, because the total energy of the collision goes into creating the top quark. It's not that the protons are being broken up and a top quark falls out,... it's that the top quark is being created out of energy; e=mc^2.

Why is it so hard for these people to say 'i don't know' and 'i/my theory was wrong'.

Because everything is a theory until it's PROVEN right or wrong. Until then, nobody is denying that we don't know. We can sit and say "i don't know" all our lives and not acheive anything, or we can do research and experimentation and prove or disprove our theories to discover new things.

Why is it so hard for these people to say 'i don't know' and 'i/my theory was wrong'.

Because everything is a theory until it's PROVEN right or wrong. Until then, nobody is denying that we don't know. We can sit and say "i don't know" all our lives and not acheive anything, or we can do research and experimentation and prove or disprove our theories to discover new things.

I think what he was trying to say was, when will the scientists step back and create a new construct upon which to base their theories. If we continue off at a tangent for years, where will that take us? The Higgs Boson fits theoretically because of other missing properties, but what's to say that these missing properties aren't due to the presence of a particle such as Higgs Boson, but due to the fact that the theory itself is flawed?

A particle that is in the range of the mass for the Higgs boson will be found. But what does that have to do with the origin of mass? Probably nothing, the origin of mass may be as simple as angular momentum.

magpies

Science truly has become religion with equations. We need to get rid of these "articles of faith" (aka assumptions).

I'm no expert, but it's my understanding this branch of science is far more concerned with finding a model that works than the actual physics involved. To this point we have little reason to doubt the standard model or super symmetry for that matter. If you have some concrete objection to the standard model, let's hear it. Random pontificating on the shortcomings of modern science is tiring, however.

Oh good. That means all the decades of worrying about nuclear Armageddon were pointless, and we shouldn't worry any more. Also, the Sun doesn't really shine. There are no stars in the sky. GPS is an illusion. Atomic clocks are fake. And all those particles physicists see when protons collide (with total mass greater than the mass of two protons at rest), are actually an illusion and don't really exist.

I have a fundamental question. Why does science insist on assigning physical properties to sub atomic particles, when they are really only observing the physical manifestations of pure energy in a state of flux? Why cant we direct our mathematics toward the flux, instead of always trying to pretend that we can find the smallest possible object and therefore know everything about the universe? I truly dont believe in quarks or gluons. http://www.physfo...ic=14275

I'm no expert, but it's my understanding this branch of science is far more concerned with finding a model that works than the actual physics involved.

What is "actual physics" if not "a model that works"? Indeed, most of the things we consider "real" and "tangible" are little more than useful simplifications or outright illusions: there is no spoon, Neo, because the spoon is actually a gigantic collection of atoms, which are in turn a gigantic collection of fields and particles and mostly empty space... And there is no such object as a rainbow, and you'll never reach a place where one either begins or ends no matter how far you travel.

Of course, subatomic physics does tend to emphasize math and imaginary numbers and extra dimensions and probabilistic modeling, over mechanics. What gears turn when an electron orbits a proton? We may never know...

They are using a brute force process of elemination. The top quark is the heaviest non divisible particle in theory and the up quark is the lightest. The top quark degrades into an up quark. So the boson (force) in this case force of mass (theorized Higgs boson) should be close to a multiple of the difference, maybe. IMHO everything in nature comes in mismatched pairs - there are probable two mass forces out there we are unaware of. this would be a first for a an odd number to appear -- but hey a God particle can do what it wants right -- or maybe it pairs with a gravity force and we can complete the GUTS.

From Standard model follows, the product of Higgs boson Yukawa coupling to the left- and right-handed top quarks have nearly the same rest mass (173.1±1.3 GeV/c2) like those predicted for Higgs boson (178.0 ± 4.3 GeV/c2). We can compare the way, in which Higgs is supposed to be proved and detected at LHC:

The 10 E+107 difference in predictions of relativity and quantum mechanics is definitelly NOT model, that "works".

Granted, but that's because you take those two models out of their respective domains, and try to apply them under circumstances they were clearly not designed to cover. By analogy, consider trying to apply Newtonian physics in a non-inertial reference frame: you will fail, but that doesn't mean that the Laws of Motion are suddenly a bad model.

Just don't be under the illusion that *any* "unified theory of everything" would itself be anything more or less than yet another "model that works" (under certain assumptions and preconditions.)

The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid. Some models of supersymmetry predict that the lightest Higgs boson (of several) will have a mass only slightly above the current experimental limits, at around 120 GeV or less.